My aim:
1) Good clear distortion-free sound across the entire spectrum that's easy to listen to and appreciate with the roof down at 85mph.
2) Enough future-proof connectivity options and full integration of iphone/sat nav handsfree etc.
3) Keep everything looking as if Porsche had installed it themselves.
Simple really

I'll tackle point 2 first. I'm about to upgrade the Stereo a Pioneer AVIC double DIN all-singing all dancing unit and have bought a mounting kit made in the 'states that should integrate it perfectly into the dash. I've managed to track down a 'no smoking' hole-filler from a 996 which integrates into the dash perfectly..and will have a USB slot behind it. Coupled to the built in SD slot and DVD (which also plays DVD audio) and the full control of an Ipod from the central cubby not to mention Parrot Bluetooth, DAB, Freeview and Full European 3D satNav...that's all bases covered.
Till that goes in, I've got a Becker Traffic Pro: It's a single DIN unit that matches the dash exactly, but has the added advantage of European Sat Nav, Aux-in for an MP3 player and the best radio Tuner I've ever heard.
So, the Speakers. I've gone with Infinity Kappas for a number of reasons: This isn't a Sales pitch but suffice to say if you're using a standard factory wiring loom and modest power levels, then these are not only reasonably priced, but offer a very good sound.
The current set up is a pair of 4" Co-Ax's in the dash tops, a pair of 6.5" co-ax's in the doors and a single 6.5" Sub. 50Watts RMS into each of the 4 satellite drivers and 150w RMS into the Subwoofer - a CDT Audio 6.5+, chosen because of the sound quality - and quantity that can be produced by such a small speaker in a very small box. (7 litres IIRC)
I've said at the moment because I'm so impressed with the CDT Audio Sub, that I may be going for a 3-way CDT Audio system - 6" Mid Bass in the door, 4" Mid range in the dash and 1" silk tweeter in the A Pillar...
Dash top installs are fairly straightforward if you get a speaker that's shallow enough and can utilise the mounting ring from the original speaker - which you have to destroy to utilise.

Here you can clearly see the difference in the size of the magnet and the tweeter -

(as in there is one). Honestly for a Prestige Marque the standard fit speakers are 'king shocking.
So the bit you need to keep hold is the fitting ring that your 4" mounts to -

- and once mounted:

Once re-fitted, I prised off the Infinity badges from the (unused) grills and stuck them on the dash-top grills a la Bose system.